A typical gas turbine engine has an annular axially extending flow path for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The compressor section includes a plurality of rotating blades which add energy to the working fluid. The working fluid exits the compressor section and enters the combustion section. Fuel is mixed with the compressed working fluid and the mixture is ignited to add more energy to the working fluid. The resulting products of combustion are then expanded through the turbine section. The turbine section includes another plurality of rotating blades which extract energy from the expanding fluid. A portion of this extracted energy is transferred back to the compressor section via a rotor shaft interconnecting the compressor section and turbine section. The remainder of the energy extracted may be used for other functions.
Recent gas turbine engine development has resulted in rotor blades having more effective and efficient interaction with fluid flowing in the flow path. This has resulted in fewer rotor blades per disk and a lighter rotor assembly. As a consequence of having fewer rotor blades per disk, the spacing between adjacent rotor blades has increased.
Each of the rotor blades includes an airfoil portion, a root portion, and a platform. The airfoil portion extends through the flow path and interacts with working fluid to transfer energy between the rotor blade and working fluid. The root portion engages the attachment means of the disk. The platform typically extends laterally from the rotor blade to a platform of an adjacent rotor blade. The platform is disposed radially between the airfoil portion and the root portion. The platform includes a radially outward facing flow surface. The plurality of platforms extends circumferentially about the longitudinal axis of the gas turbine engine to define a radially inner flow surface for working fluid. This inner flow surface confines working fluid to the airfoil portion of the rotor blade.
Platforms are generally of two types. The first is a chevron type which includes lateral edges curved to approximate the airfoil shape of the rotor blade. This type of shape minimizes the lateral extension of the platform from the rotor blade. Minimizing the lateral extension, or cantilevered portion of the platform, minimizes the rotationally caused bending stress in the platform.
The second type of platform includes parallel lateral edges which extend linearly in an axial orientation. Parallel edges provide for ease of manufacture of the rotor blades and ease of installation onto the disk. However, the parallel lateral edges result in platforms which extend further outward from the blade. The lateral extension of the platform becomes more significant as the spacing between adjacent rotor blades increases. The combination of parallel, linear edges and increased rotor blade spacing results in a platform having a significant cantilever.
As a result of the lateral extension, this type of platform has higher bending stress than a comparable chevron platform. The bending stress is particularly significant in the region of the attachment of the platform to the root portion and airfoil portion of the rotor blade. To accommodate this stress, the parallel edged platform is typically made thicker, in the radial dimension, with a lateral taper. Increasing the thickness of the platform adds to the bulk of the blade and adversely affects operating efficiency of the gas turbine engine.
Another concern associated with the parallel edged platforms is the overheating of the platforms. The laterally outermost portion of the platforms receive little cooling from the core cooling fluid passing through the rotor blade. Therefore this portion of the platform is subject to degradation as a result of overheating. Degradation of the platform reduces the effectiveness of the platform to confine the flow of working fluid to the airfoil portion of the blade and thereby causes a reduction in operational efficiency of the gas turbine engine.
A solution to the overheating of the platform is to provide cooling fluid to the platform. Typically this involves having cooling holes pass radially through the platform to a damper cavity located underneath the platform. The damper cavity contains cooling fluid which has passed through various passages within the rotor assembly to provide cooling to the rotor assembly. This cooling fluid then passes out through the cooling holes and cools the platform in the vicinity of the cooling holes.
A problem associated with cooling holes of this type has been to properly locate them. Due to pressure fluctuations over the surface of the platform, the pressure differential between the damper cavity and the flow path fluctuates along the surface of the platform. This may lead to a negative pressure differential near the cooling hole and may cause ingestion of working fluid through the cooling hole and into the damper cavity. Within the damper cavity the ingested working fluid heats up the cavity and may cause degradation to nearby structure, including the platform, the damper, and the disk attachment area. In addition, non-gaseous products in the working fluid may block the cooling holes and reduce or prevent cooling fluid from exiting the damper cavity, thereby blocking cooling fluid from flowing over the flow surface of the platform.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop effective cooling means for rotor 0lade platforms.